Hearing aid fitting systems are well known in the art, e.g. cf. U.S. Pat. No. 5,835,611. Such systems have proven adequate for adjusting signal processing parameters so that the hearing aid accurately compensates the actual hearing loss of the hearing aid wearer.
The fitting process typically involves measuring the auditory characteristics of an individual's hearing, estimating the acoustic characteristics needed to compensate for the particular auditory deficiency measured, adjusting the auditory characteristics of the acoustic hearing aid so that the appropriate acoustic characteristics may be delivered, and verifying that these particular auditory characteristics do compensate for the hearing deficiency found by operating the acoustic hearing aid in conjunction with the individual.
Standard techniques are known for these fittings which are typically performed by an audiologist, hearing aid dispenser, otologist, otolaryngologist, or other doctor or medical specialist.
In the well-known methods of acoustically fitting a hearing aid to an individual, the threshold of the individual's hearing is typically measured using an audiometer, i.e. a calibrated sound stimulus producing device and calibrated headphones. The measurement of the threshold of hearing takes place in a room with very little audible noise.
Generally, the audiometer generates pure tones at various frequencies between 125 Hz and 8,000 Hz. These tones are transmitted to the individual being tested, e.g. through headphones of the audiometer. Normally, the tones are presented in step of an octave or half an octave. The intensity or volume of the pure tones is varied and reduced until the individual can just barely detect the presence of the tone. This intensity threshold is often defined and found as the intensity of which the individual can detect 50% of the tones presented. For each pure tone, this intensity threshold is known as the individual's air conduction threshold of hearing. Although the threshold of hearing is only one element among several that characterizes an individual's hearing loss, it is the predominant measure traditionally used to acoustically fit a hearing aid.
Once the threshold of hearing in each frequency band has been determined, this threshold is used to estimate the amount of amplification, compression, and/or other adjustment that will be employed to compensate for the individual's loss of hearing. The implementation of the amplification, compression, and/or other adjustments and the hearing compensation achieved thereby depends upon the hearing aid being employed. There are various formulas known in the art which have been used to estimate the acoustic parameters based upon the observed threshold of hearing. These include generic rules, such as NAL and POGO, which may be used when fitting hearing aid from most hearing aid manufactures. There are also various proprietary methods used by various hearing aid manufacturers. Additionally, based upon the experience of the person performing the testing and the fitting of the hearing aid to the individual, these various formulas may be adjusted.